An organic electroluminescent device (hereinafter referred to as organic EL device) in the simplest structure is generally constituted of a light-emitting layer and a pair of counter electrodes holding the light-emitting layer between them. The organic EL device functions by utilizing the following phenomenon; upon application of an electric field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer with emission of light.
In recent years, studies have been started to develop organic EL devices in which organic thin films are used. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been directed toward practical applications to high-performance flat panels featuring self-luminescence and high-speed response.
Further, in an effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of Alq3 and many others have utilized fluorescence. However, the utilization of phosphorescence, that is, emission of light from the triplet excited state, is expected to enhance the luminous efficiency three to four times that of the conventional devices utilizing fluorescence (emission of light from the singlet excited state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer was investigated, but these derivatives merely produced luminance at an extremely low level. Europium complexes were also investigated in trials to utilize the excited triplet state, but they too failed to emit light at high efficiency. In recent years, as stated in patent document 1, a large number of researches are conducted on phosphorescent dopant materials, with a focus on the use of organic metal complexes such as iridium complexes, for the purpose of enhancing the luminous efficiency and extending the life.